1. Field of the Invention
This invention relates to a conductive rubber roller used in image forming apparatus such as an electrophotographic copying apparatus, a printer and an electrostatic recording apparatus, and more particularly to a conductive rubber roller used as, e.g., a transfer roller of a transfer assembly by means of which a tansferable image based on a toner image is transferred to a recording medium such as paper; the toner image being formed and held on an image bearing member such as an electrostatic photosensitive member by an imaging means such as an electrophotographic process or an electrostatic recording process.
2. Related Background Art
In the image forming apparatus such as an electrophotographic copying apparatus and an electrostatic recording apparatus, a contact charging system is prevalent in which a conductive rubber roller to which a voltage is kept applied is pressed against the surface of an electrostatic photosensitive member to charge it electrostatically. Around the electrostatic photosensitive member (drum), which the heart of image formation, conductive rubber rollers are independently used in individual steps of charging, transfer and so forth.
In recent years, as rubber components of such conductive rubber rollers, polar rubbers such as acrylonitrile-butadiene rubber and epichlorohydrin type rubber are used. The polar rubbers have conductivity (ionic conductivity) because of the presence of a polar group in the polymer, and are known to be suitable for the conductive rubber rollers because of less scattering of electrical resistance and a small voltage dependence of the electrical resistance.
Elastic-material layers of the conductive rubber rollers are required to have a volume resistivity of 2×109 Ωcm or less in many cases. Where the rubber component is acrylonitrile-butadiene rubber alone, its vulcanized product has a volume resistivity of approximately from 2×109 to 1×1010 Ωcm, resulting in an insufficient conductivity. Also, the acrylonitrile-butadiene rubber has an inferior ozone resistance, and hence no sufficient electrification durability is achievable.
Accordingly, a method is commonly used in which an epichlorohydrin type rubber, which is known to have a volume resistivity of approximately from 1×107 to 3×109 Ωcm, is blended with the acrylonitrile-butadiene rubber to make control so that the desired volume resistivity can be achieved (see, e.g., Japanese Patent No. 3656904).
Also in recent years, conductive rubber rollers having lower electrical resistance are required in order to make adaptation to color image formation and high-quality image formation, and a method is also used in which the epichlorohydrin type rubber is used alone or an ionic conducting agent of various types such as a quaternary ammonium salt which contains perchlorate ions or chloride ions is added (see, e.g., Japanese Patent Application Laid-open No. 2002-132020).
However, in the case of the conductive rubber rollers making use of such a rubber elastic material, their resistivity may change depending on variations of environmental factors such as temperature and humidity, and hence there is a problem that the image quality changes depending on service environment. In particular, the epichlorohydrin type rubber has problems that it tends to be influenced by humidity to greatly cause environmental variations of resistivity. The method in which an ionic conducting agent of various types such as a quaternary ammonium salt is added also has a possibility of causing changes with time of resistivity because of contamination, electrification or the like due to surface migration of ions. Hence, any electrification durability adaptable to high-speed and long-lifetime image formation can not be achieved in some cases.
In the case when the epichlorohydrin type rubber and the quaternary ammonium salt which contains perchlorate ions or chloride ions are mixed, there are also possibilities of making compression set occur very seriously and besides causing poisonous gas and dioxin at the time of incineration, for the reasons that, e.g., the chlorine causes side reaction.
Accordingly, as a method by which the problems as stated above can be resolved, a method is variously attempted in which an ethylene oxide-propylene oxide-allyl glycidyl ether terpolymer is mixed in a stated quantity in the polar rubber such as acrylonitrile-butadiene rubber or epichlorohydrin rubber. The ethylene oxide-propylene oxide-allyl glycidyl ether terpolymer contains ether-oxygen linkages and hence has the function to stabilize metallic cations and the like in the polymer to lower electrical resistance. The ethylene oxide-propylene oxide-allyl glycidyl ether terpolymer can also cross-link with other rubbers because it has a large polarity and a superior compatibility with other polar rubbers and also because the allyl glycidyl ether has unsaturated bonds. This can not easily cause bleeding and electrostatic photosensitive member contamination as being different from conducting agents such as quaternary ammonium salts. Also, the ethylene oxide-propylene oxide-allyl glycidyl ether terpolymer contains no halogen element, and hence it is reported that it has no problem of the side reaction of chlorine the epichlorohydrin type rubber may cause and good rubber materials can be obtained which are well preventive of compression set (see, e.g., Japanese Patent Application Laid-open No. 2002-105305).
It is also reported that an ethylene oxide-propylene oxide-allyl glycidyl ether terpolymer having a specific copolymerization ratio is mixed in a stated quantity to thereby obtain a conductive rubber roller having a lower electrical resistance and having a good ozone resistance and a smaller environmental dependence than the conductive rubber roller making use of a rubber composition comprising a blend of acrylonitrile-butadiene rubber and epichlorohydrin type rubber and the conductive rubber roller making use of a rubber composition to which an ionic conducting agent is added, having been conventionally used (see, e.g., Japanese Patent No. 3600517 and Japanese Patent Application Laid-open No. 2006-037042).
In the case of the rubber composition disclosed in Japanese Patent No. 3600517, a conductive rubber roller having a superior ozone resistance can be obtained. However, further improvement has been sought in regard to the achievement of low electrical resistance, the environmental dependence and the anti-contamination to photosensitive member. In the case of the rubber composition disclosed in Japanese Patent Application Laid-open No. 2006-037042, a conductive rubber roller having a small environmental dependence can be obtained. However, it is difficult in some cases to achieve the low electrical resistance and the small environmental dependence, and further improvement has been sought.